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shelf clouds
Falling rain drags with it cold air (a downdraft), which splashes out upon hitting the ground, as one might expect paint to do if poured from a paint tin.  This splashing is the gusts of wind you are blasted with prior to the coming of a storm.  Of course, that cold air is not running into empty space; there is a layer of warm air hanging at ground level which is violently forced up out of the way by the rushing cold gusts.  Those two differently heated layers rub along each other, cooling the moisture contained in the warm layer and causing it to condense.  The product of this condensation is called a shelf cloud.
I became interested in this after being bowled over by the amazing closing images from this sequence of photographs.
Here are some other amazing shelf cloud shots:
australiasevereweather.com/photography/photos/2005/index16.html
stormtrack.org/jensen/1.htm
australiasevereweather.com/photography/photos/2003/1002bv25.jpg
australiasevereweather.com/photography/photos/2003/1002bv24.jpg
This next one is really quite amazing.  The photographer, one mr anthony cornelius, must have been brave; i'd have run for my life from that monster . The website he posts on is worth a look.